1. Field of the Invention
In its more general aspect, the present invention relates to manufacturing electronic semiconductor devices.
More particularly, the present invention relates to a method for manufacturing electronic semiconductor devices by providing circuit patterns on a semiconductor substrate through either engraving or etching of said substrate with watery acid solutions.
2. Description of the Related Art
As it is well known, one of the fundamental technological steps for manufacturing electronic semiconductor devices is manufacturing circuit configurations or patterns on a semiconductor substrate.
At this purpose, a photo-resist layer is deposited on the semiconductor substrate and it is afterwards selectively removed, through traditional photolithography, in order to expose portions of said substrate according to a defined geometry corresponding to the desired circuit pattern. Then, the substrate is selectively etched in correspondence with the exposed portions in order to transfer the defined geometry from the photo-resist layer to one or more underlying semiconductor substrate layers.
In an embodiment of this method, according to the typology and features to be assigned to the final product, it can be convenient or necessary to manufacture circuit patterns on a semiconductor substrate by etching it with watery solutions containing acids. In this case, a photo-resist material of the so-called “i-line” type is generally applied in advance on the semiconductor substrate, i.e., a photo-resist material for which the exposure to wavelengths (A) of about 365 nm in the common photolithographic development is provided.
Although advantageous under many aspects, this embodiment has the drawback that through the photolithographic development applied to the “i-line” photo-resist material it is impossible to control adequately the layout, overlay and current density features to be obtained in the final electronic device if the latter must be manufactured with very reduced submicromic size and thus with very thick circuit structures.
However it is known how microelectronics, for several years, has undergone a general trend providing a steady reduction of semiconductor device sizes and consequently a continuous thickening of the various circuit structures forming them.
According to this more and more urgent need, methods for manufacturing electronic semiconductor devices have been thus developed in the prior art, which provide, in the domain of circuit pattern manufacturing, the use of so-called “deep UV” photo-resist materials instead of common “i-line” photo-resist materials, whose exposure during the common photolithographic development is performed in the far ultraviolet (λ=248 nm).
The use of “deep-UV” photo-resist materials allows innovative equipment to be used, particularly photo-resist exposure machines operating in the far ultraviolet, through which an adequate layout, overlay and current density control can be obtained, even when manufacturing electronic semiconductor devices having a very critical submicrometric size.
Although advantageous under many aspects, these methods have however the drawback of being scarcely usable when in the production line an etching of the semiconductor substrate is provided with acid, watery solutions in correspondence with the exposed portions thereof by selectively removing the “deep-UV” photo-resist material previously applied thereon.
In fact it has been verified that when particularly aggressive watery acid solutions are used for etching the semiconductor substrate, a loss of adhesion between the “Deep UV” photo-resist layer and the substrate to be etched disadvantageously occurs, and subsequently the etching reaction can occur also in the unexposed areas of the semiconductor substrate at the interface with the above photo-resist layer.
The causes determining this loss of adhesion must be mainly searched in the “deep UV” photo-resist material own features and in the relevant photolithographic development process, as well as in the semiconductor substrate hydrophilicity.
In fact, as it is well known, a “deep UV” photo-resist material essentially comprises a resin containing protective groups which can be removed by means of an acid, a photosensitive compound effective to generate an acid when exposed to an ultraviolet radiation (Photo Acid Generator—PAG) and a solvent.
The adhesion between the photo-resist material and the substrate is mainly due to the interaction between the polar groups of the resin and those of the substrate.
The polar groups of the resin, which generally consists of a poly-hydroxy-styrene derivative, are essentially composed of a limited number of free hydroxyl groups (—OH) and in large part of carbonate groups (OCO2) of the protective groups (for example ter-butyl-carbonate) used to block some resin hydroxyl groups in order to make it essentially insoluble in watery solutions.
During the common photolithographic development, the substrate whereon the “deep UV” photo-resist material has been applied and adhered is selectively exposed, through a convenient mask, to ultraviolet radiation and it thus undergoes a baking step (Post Exposure Bake—PEB) and a development step.
During the exposure, the photo-resist compound generates ions H+ (acid), which in the baking step remove in the exposed areas the resin protective groups (in this specific case by etching the group OCO2) generating a soluble compound in a basic solution used for the development step. FIG. 1 shows a reaction diagram concerning the above-described photolithographic process.
Now, since the reaction between ions H+ and resin protective groups can partially occur also at room temperature, when very aggressive acid solutions are used for etching the substrate after the photolithographic process, it may occur that the photo-resist material on substrate unexposed areas is etched by the ions H+ of the acid solution being used, and the resin carbonate polar groups are subsequently partially or totally removed in the same way as shown in the reaction diagram of FIG. 1.
Therefore, the resin deprived of the carbonate polar groups would no more be able to adhere efficiently to the substrate and, thus, the substrate etching can also extend to the regions thereof being unexposed to the photolithographic process.
Moreover, it must be noted that the substrate etching in unexposed regions is further favored by the semiconductor substrate hydrophilicity which favors the diffusion of the acid solution at the interface between the substrate and the photo-resist material.
All this clearly involves an inadequate control of the size of the circuit patterns formed on the semiconductor substrate, to the total detriment of the functionality and reliability of final electronic semiconductor devices which are unsatisfactory mainly when these devices are manufactured with very reduced size.
In prior art it has been tried to improve the adhesion between the “deep UV” photo-resist materials and the substrate by treating surface of the latter with hexamethyidysilazane (HMDS). Nevertheless, this treatment is ineffective when, after the “deep UV” photo-resist material development, the substrate is chemically etched with very aggressive watery acid solutions.